This depends on what you want to do with the login screen. You can configure some basic options in the Login Screen Settings app (System/Administration/Login Screen) - whether a user should be automatically logged in, the default session etc.
However to change the login screen more graphically Ubuntu Tweak can be used to change the background, logo etc.
Use this command in the terminal:
Logout, and you will be prompted with the Appearance window. change the theme as you change it for the desktop, and login back, and type this command:
That's it.
This depends on what you want to do with the login screen. You can configure some basic options in the Login Screen Settings app (System/Administration/Login Screen) - whether a user should be automatically logged in, the default session etc.
However to change the login screen more graphically Ubuntu Tweak can be used to change the background, logo etc.
U can use GDMtweak to change theme and Icon. see the link http://www.webupd8.org/2011/05/change-gdm-theme-background-in-ubuntu.html
I'm not aware of a graphical tool to automate the process but is not overly complicated to change the theme of gdm.
Take a look at Gnome-Look GDM, most of the themes there come with good instructions on how to install them.
There is a graphical tool for this. It's called GDM2Setup. You can get it from this PPA: https://launchpad.net/gdm2setup
Theme Support for GDM has been terminated since UBUNTU 9.Sorry..!!
/usr/share/gnome-shell/theme
folder.Run
Search for the following section
Change the name of the image to your image and set background to repeat or no-repeat.
Save the file.
This works for Ubuntu GNOME, for other flavours it might not work.